Bolt torque calculator
Torque is a very indirect way of setting preload: most of it is spent on friction. The nut factor bundles all of that into one number, and it varies a lot.
Class 8.8 is 640 MPa, 10.9 is 940, 12.9 is 1100.
Dry steel 0.2; lightly oiled 0.15; zinc plated 0.22; PTFE coated 0.12.
- In pound-feet
- 41.06
- Clamp force
- 27 835
- Proof load of the bolt
- 37 113
- Torque if lightly oiled instead
- 41.75 N m
- Bolt strain
- 0.0024
- Roughly this share of the torque is friction
- 85 %
Show the working
- Bolt diameter = 10
- Pitch = 1.5
- Proof strength = 640
- Preload target = 75
- Nut factor K = 0.2
- stress_area = pi/4 * (10 - 0.938194 * 1.5)^2 = 57.9896
- Proof load of the bolt = 57.9896 * 640 = 37 113.3
- Clamp force = 37 113.3 * 75/100 = 27 835
- Tightening torque = 0.2 * 10/1000 * 27 835 = 55.67
- In pound-feet = 55.67 / 1.3558179483314004 = 41.0601
- Bolt strain = 27 835 / (57.9896 * 200000) = 0.0024
- Roughly this share of the torque is friction = 85 = 85
- Torque if lightly oiled instead = 0.15 * 10/1000 * 27 835 = 41.7525
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
Scenarios & local history
Compare a saved scenario
Recent calculations
The formula
- T = K D F, with D the nominal diameter in metres and F the preload
Around 85 to 90% of tightening torque is consumed by friction under the head and in the threads. That is why torque-to-preload scatter is typically plus or minus 25%.
Lubricating a bolt torqued to a dry-value specification will over-tension it, often past yield.
Angle-controlled or stretch-controlled tightening is far more accurate than torque when it matters.